1. Field of the Invention
The present invention relates to a method for producing aldehydes by subjecting an olefinic unsaturated compound to a hydroformylation reaction in the presence of a Group 8 metal-phosphite complex catalyst.
2. Discussion of Background
A process for producing aldehydes which comprises reacting an olefinic unsaturated compound with carbon monoxide and hydrogen in the presence of a Group 8 metal complex catalyst, is widely practiced on an industrial scale. As a catalyst for this hydroformylation reaction, it is common to employ a complex catalyst having a Group 8 metal such as rhodium modified with a ligand such as a trivalent phosphorus compound. In order to improve the activity or selectivity for the hydroformylation reaction, various ligands have been studied. For example, Japanese Examined Patent Publication No. 10730/1970 discloses that a rhodium catalyst modified with a trivalent phosphorus ligand such as a triaryl phosphine or a triaryl phosphite, is effective.
Among various catalysts, a catalyst modified with a phosphite ligand is known to show a high catalytic activity and excellent selectivity in the hydroformylation reaction.
However, as disclosed in Japanese Unexamined Patent Publication No. 51229/1984, with a phosphite ligand such as triphenyl phosphite, it is known that the ligand is relatively quickly decomposed in the hydroformylation reaction system, whereby the catalytic activity will decrease, and it is therefore necessary to continuously supplement the phosphite ligand. Accordingly, for the purpose of not only improving the activity and selectivity of the catalyst but also minimizing the decrease of the catalytic activity due to the loss of the phosphite ligand, various phosphite ligands have been proposed.
For example, Japanese Unexamined Patent Publications No. 51228/1984 and No. 290551/1992 disclose that the stability of the phosphite itself is improved in order to suppress the decomposition of the phosphite ligand. Further Japanese Unexamined Patent Publication No. 156636/1985 discloses a method wherein a tertiary amine is present in the hydroformylation reaction zone to stabilize the phosphite ligand.
Further, with respect to a technique for separating the catalyst, Japanese Unexamined Patent Publication No. 49190/1975 discloses an adsorption method, Japanese Unexamined Patent Publication No. 122948/1982 discloses a precipitation method, and Japanese Unexamined Patent Publication No. 231435/1990 discloses a membrane separation method. Japanese Unexamined Patent Publication No. 2994/1981 discloses a method wherein a catalyst solution obtained by a hydroformylation reaction using a rhodium-phosphine complex catalyst, is contacted with a paraffin or a cycloparaffin and a polar organic solvent, followed by separation into two phases, so that the majority of the rhodium complex can be obtained in the polar organic solvent phase. However, industrially, it is most common to employ distillation for the separation.
As a reaction medium for a usual liquid phase hydroformylation reaction, it is common to employ not only an inert solvent such as an aromatic hydrocarbon but also a high boiling by-product produced by a condensation reaction of an aldehyde product. When either medium is employed, the high boiling by-product will be formed as time passes and will gradually accumulate unless it is withdrawn. The accumulation of such a high boiling by-product brings about not only a mass balance problem but also a necessity to withdraw a part of the high boiling by-product continuously or intermittently out of the system, since it sometimes becomes poisonous to the catalyst.
On the other hand, in a case where a Group 8 metal complex catalyst is used on an industrial scale, it is necessary to recycle the catalyst continuously, since the Group 8 metal is expensive. Accordingly, when a part of the high boiling by-product is purged as described above, it is necessary to selectively withdraw a high boiling by-product in order to minimize the loss of the Group 8 metal complex catalyst. As a separation method for withdrawing such a high boiling by-product, distillation has been commonly used in the conventional phosphine type process.
When a commonly employed aromatic hydrocarbon solvent such as benzene, toluene or xylene is used as the solvent for a hydroformylation reaction, the boiling point of the catalyst solution becomes very high. Accordingly, steam distillation may sometimes be used as a means for lowering the temperature of the distillation still, when distillation is employed for the separation of the high boiling by-product and the catalyst component. The temperature of such a distillation still will be 120.degree. C. or as high as 170.degree. C. or higher in some cases.
When an organic phosphine compound such as triphenyl phosphine is used as the ligand of the Group 8 metal catalyst in the hydroformylation reaction, there will usually be no problem even under a high temperature condition as mentioned above. Whereas, when a phosphite type ligand which exhibits a high activity and high selectivity in the reaction as compared with a phosphine type ligand, is to be used, such a phosphite is substantially inferior in the thermal stability to the phosphine. Accordingly, in a method for separating a high boiling by-product by distillation at a high temperature as mentioned above, rapid decomposition of the phosphite takes place. Thus, it has been practically impossible to use a phosphite for an industrial operation.
Further, in a case where steam distillation is used to lower the temperature, vigorous decomposition of the phosphite takes place, since the phosphite is highly susceptible to hydrolysis. As the phosphite is highly susceptible to hydrolysis, it has been believed that water in the process accelerates decomposition of the phosphite, and the water should be reduced as far as possible. On the other hand, as disclosed in Japanese Unexamined Patent Publication No. 199728/1994, presence of a small amount of water is advantageous. Even then, it is described that to add too much water, e.g. water in such an amount that an aqueous phase and an organic phase are separated during the extraction separation operation, is undesirable. Namely, in the prior art, it has been difficult to separate the complex catalyst or the high boiling by-product by an aqueous extraction operation in the phosphite type process.
Accordingly, to utilize the phosphite type ligand having higher performance than the phosphine type ligand industrially, it has been an important object to establish a method whereby decomposition of the phosphite ligand is suppressed and a part of the high boiling by-product is withdrawn while minimizing the loss of the complex catalyst.